1. Field of the Invention
This invention concerns a method of manufacturing a semiconductor laser. More particularly, this invention concerns a method of manufacturing a window structure semiconductor laser.
2. Description of the Prior Art
Once the optical power density at mirrors of a semiconductor laser exceeds several MW/cm2, the mirrors suffer from Catastropic Optical Damage (COD), which is a local destruction caused by local heating resulting from absorption of the laser light by an active layer. Thus, the maximum available optical power of a semiconductor laser has been limited by COD.
However, this limitation has been overcome by an improved structure called a "window structure", in which non-absorbing regions called "window" are provided near the mirror so that the local heating may be reduced.
FIG. 1 shows a conventional window structure semiconductor laser disclosed in "Shuuki Ouyou Butsuri Gakkai Kouen Yokoushu, 12a-ZR-8" (Extended Abstracts 12a-ZR-8, The 48th Autumn Meeting, 1987, The Japan Society of Applied Physics and Related Societies in Japan). In this drawing, the portion illustrated by a dotted line is partially cutaway for convenience of illustration. In this structure, an active layer within about 30 .mu.m from the mirror is removed to eliminate the absorption of the laser light and to form the window.
The semiconductor laser of FIG. 1 comprises a semiconductor substrate 11 made of III-V compound semiconductor mixed crystal, a current blocking layer 12, a lower cladding layer 13, an optical waveguide layer 14, an active layer 15, a first upper cladding layer 16, an antioxidizing layer 17, a second upper cladding layer 18 and an ohmic contact layer 19. Furthermore, the laser includes upper electrode layer 20 on the ohmic contact layer 19 and a lower electrode layer 21 under the substrate 11.
FIGS. 2A to 2F show a method of manufacturing the structure shown in FIG. 1. At first, a semiconductor substrate 11 made of P-type GaAs is prepared. The main surface of the substrate is partially etched to form a mesa 11A of about 1 .mu.m height. (FIG. 2A)
Next, a current blocking layer 12 made of N-type GaAs is formed on the substrate 11 by a liquid phase epitaxial method. (FIG. 2B) Due to the characteristic of the epitaxial method, the surface of the current blocking layer 12 becomes flat.
Then, an etching is carried out to form a mesa 12A of 1 .mu.m height and 20 .mu.m width. (FIG. 2C)
Next, an etching is carried out to form a groove 12B partially exposing the mesa (shown as 11B) of the semiconductor substrate so as to form a current path. (FIG. 2D)
Then, a lower cladding layer 13 made of P-type Al.sub.0.41 Ga.sub.0.59 As having 0.2 .mu.m thickness, an optical waveguide layer 14 made of P-type Al.sub.0.31 Ga.sub.0.69 As of 0.15 .mu.m thickness, an active layer 15 made of P-type of N-type Al.sub.0.08 Ga.sub.0.92 As of 0.06 .mu.m thickness, a first upper cladding layer 16 made of N-type Al.sub.0.41 Ga.sub.0.59 As and an antioxidizing layer 17 made of N-type Al.sub.0.15 Ga.sub.0.85 As are successively formed by the liquid phase epitaxial method. (FIG. 2E)
Next, an etching is performed to partially remove the antioxidizing layer 17, the first upper cladding layer 16 and the active layer (15) at the vicinity of the emitting facet, corresponding to the area 22 and 23 in FIG. 1. (FIG. 2F)
Then, a second upper cladding layer 18 made of N-type Al.sub.0.41 Ga.sub.0.59 As and an ohmic contact layer 19 made of N-type GaAs are formed successively by Metalorganic Chemical Vapor Deposition (MOCVD) method.
After an upper metallic electrode 21 and a lower metallic electrode layers 20 are formed, the wafer thus formed is cleaved to produce a semiconductor laser having mirror surfaces 25 and 26 (See FIG. 1).
In this construction, the cladding layer 13 is made so thin outside the groove 12B that the laser light, which is guided along the optical layer 14, penetrates into the current blocking layer 12 and suffers from the losses. This results in mode confinement just above the groove 12B and provides a waveguide parallel to the junction plane. This waveguide is classified to a complex index guide by its waveguiding mechanism.
A stable single lateral mode operation, which is essential for practical use of laser diodes, is general achieved by restricting the width W of the waveguide to a critical value. This critical value is about 5 .mu.m for the complex index guide of this prior art, in which the width of the waveguide is equal to the width of the groove 12B. within this restriction, W is designed as wide as possible because the available maximum power from the laser diodes is proportional to W. Taking account of the maximum width-error of about 1 .mu.m on making the groove 12B by conventional etching through a photoresist mask, the designed value of W is eventually 4 .mu.m.
This conventional etching technique is used also in making the window region as previously explained in FIG. 2F. This etching has to stop just at the interface between the active layer 15 and the optical guide layer 14. Otherwise, significant absorption would remain in the window (underetching case) or optical scattering, caused by decoupling between the thinner optical guide layer in the window and the thicker optical guide layer in the inner region, would deteriorate the lateral mode (overetching case).
It is concluded that this conventional window structure, both the waveguide and windows of which are made by the convenional etching technique, is not suitable for mass production because the accuracy of the etching is not high enough to make the windows.
FIG. 3 is a perspecive view of a second conventional window structure semiconductor laser, where the portion illustrated by a dotted line is partially cutaway. This construction is disclosed in "1985, Shunki Ouyou Butsuri Gakkai Kouen Yokoushu, 30p-ZB-10" (Extended Abstracts 30p-ZB-10, The 32nd Spring Meeting, 1985, The Japan Applied Physics and Related Societies).
In this laser, an active layer 34 is sandwiched by a lower optical guide layer 33 made of N-type Al.sub.0.25 Ga.sub.0.75 As layer and an upper optical guide layer 35 made of P-type Al.sub.0.25 Ga.sub.0.75 As layer 35. These layers are again sandwiched by an upper cladding layer 32 made of P-type Al.sub.0.3 Ga.sub.0.7 As layer 32 and a lower cladding layer 36 made of N-type Al.sub.0.3 Ga.sub.0.7 As. On the upper cladding layer, an oxide layer 37 made of SiO2 with a stripe shape opening of width W is provided so that a driving current injected from the upper electrode is confined just under this opening.
The active layer 34 has a multiquantum well (MQW) structure comprising five 8 nm thick wells made of P-type GaAs and 12 nm thick barriers made of Al.sub.0.2 Ga.sub.0.8 As. In the region 38, which is located just outside the opening of the oxide mask 37 and shown by hatched lines in FIG. 3, zinc is diffused and the MQW structure of the active layer 34 is disordered because the interdiffusion of column III atoms is enhanced during the zinc diffusion. This disordered (or interdiffused) MQW has a smaller index and a wider bandgap than the ordered MQW just under the opening of the oxide mask 37. Therefore, an real index of width W is formed as well as windows near the mirrors.
In the real index guide of this prior art, the wave guide width W should be less than about 2 .mu.m to achieve single lateral mode operation. However, the accuracy of zinc diffusion in the lateral direction is so poor that the wave guide width less than 2 .mu.m can hardly realized. Therefore, this second conventional window structure is also unsuitable for mass production.